The invention relates to structures and methods for maintaining objects at low temperatures at remote locations and more particularly to an apparatus and method for maintaining a low temperature about an object at a remote location for an extended period of time.
It is desirable to log boreholes in order to determine and physical properties of the rocks such as porosity and flow permeability. It is sometimes also desirable to determine the nature of cracks, fractures, and fissures in the rocks. Nuclear magnetic resonance systems are sometimes used to log boreholes for permeability and porosity. The performance of such nuclear magnetic resonance systems is enhanced by the use of a cryogenic environment. Superconducting gradiometers and magnetometers used in borehole logging would require such an environment. Data from such measurements can be used to determine potential fluid flow such as for geothermal steam or water through the fracture.
In logging boreholes it is frequently necessary to maintain a cryogenic temperature between about 2 and about 15 K. for an extended period of time, such as 3 to 6 hours, about a cryogenic instrument disposed in a borehole to enable it to be lowered up to a distance of 2 to 3 miles and operated at such a distance downhole for a sufficient period of time to make any desired measurements. Usually the instrument need not be maintained at a cryogenic temperature during withdrawal from the borehole.
One attempted procedure involved the use of a downhole closed-cycle refrigerator. The system was precooled at the surface, a cryogenic temperature being maintained downhole by running a conduit to and a vent from a liquid helium dewer. Needless to say, this was very cumbersome and very high pressure downhole precluded proper downhole venting of the helium. This technique is essentially too complex to manage for borehole depths on the order of 2 to 3 miles although it is workable in shallow boreholes.